Combat aircraft are provided with means for detecting and locating noncooperative electromagnetic emitters, e.g., radars and communication transmitters, such means being known as "threat warning and location systems." Presently existing threat warning and location systems utilize directive receiving antennas to provide for determining the approximate location of such emitters. In order to provide the required directivity over a wide band of frequencies, multiple antennas, each of five to ten wavelengths in dimension, are required. The installation of such sets of antennas to provide appropriate viewing angles and to satisfy other requirements on combat aircraft of advanced design creates substantial problems for aircraft designers.
The present invention is directed toward determining emitter threat location and velocity through use of signal processing rather than antenna directivity. Depending on the desired sector coverage, no more than four small-aperture antennas are required for emitter detection and location system operation. The required aperture of each of such antennas would be no more than a few inches. Such small-aperture antennas pose minimal problems in installation and can readily be made consistent with other requirements on combat aircraft of advanced design.
It has been shown that a series of measurements of the Doppler-shifted frequency of the signals from any emitter, including noncooperative emitters, when made from a moving receiver such as one installed on an aircraft, can provide a basis for calculating location and velocity of the emitter. In order to make the necessary frequency measurements with sufficient accuracy, the emitter should produce coherent signals. In particular, signals which are not continuous (e.g., pulsed radar) should be coherent on a pulse-to-pulse basis. Other methods, such as estimation of unshifted frequency, measurement of time-difference-of-arrival, or part-time use of directive antennas, must be used when the signals are not coherent.
The familiar equation for Doppler shift is: ##EQU1##
where
f.sub.D.sub..sub.i is the value of the i.sub.th measurement of the Doppler-shifted frequency, f.sub.D PA1 f.sub.o is the unshifted frequency of the emitter PA1 V is the speed of the moving receiver PA1 c is the velocity of light PA1 .theta..sub.i is the angle between the velocity vector of the moving receiver and the direction of arrival of the signal from the source for i.sub.th measurement
It might at first seem that a series of measurements of f.sub.D (expressed as f.sub.D.sub..sub.i ) coupled with velocity and position data would allow for solution of this equation in terms of .theta. (thus giving not only bearing but also, through additional calculations, being relational to the range to the emitter). Such, however, is not the case. The difficulty arises because the receiver lacks information concerning f.sub.o. The procedure outlined above leads to a consistently undetermined set of linear equations in terms of a conventional solution. The difficulty could be resolved by recording maximum and minimum values of f.sub.D while the receiver executes a 360.degree. turn. However, such a method is considered to be totally unacceptable from an operational standpoint.
It is therefore desirable to provide a method for determining the range between a receiver and an emitter of electromagnetic energy of unknown frequency.